1. Field of the Invention
This invention relates to a method for using bacteria to produce hydrogen gas, and more particularly this invention relates to a method for using thermophilic bacteria to generate hydrogen gas from a wide variety of feedstocks.
2. Background of the Invention
Hydrogen gas holds promise as the fuel of the future. The U.S. currently consumes 3.6 trillion cubic feet (TCF) of hydrogen gas annually, with a worldwide consumption of about three times that amount. The U.S. demand alone is expected to increase by 40% to 5.0 TCF in the next five years.
Most of the hydrogen gas produced throughout the world is made from synthesis gas generated either by reformation of natural gas or from the gasification of coal. Not only are these processes costly, but they are hostile to the environment. Furthermore, these methods use fossil fuels, which are non-renewable.
Biological processes have been used previously to generate hydrogen gas. Biological processes are particularly attractive because renewable feedstocks (i.e., biomass, even organic waste streams) are utilized. Research in this area has concentrated on three different approaches:
1) Using photosynthetic organisms to split water;
2) Using fermentative bacteria to digest hydrocarbons;
3) Using a combination of bacteria in which some of the bacteria digest the complex hydrocarbons to make an appropriate feedstock for the hydrogen-producing bacteria.
However, and as discussed more fully, infra, these efforts have not yet resulted in commercial success because of one or more problems. In some cases, the metabolic processes that produce hydrogen gas are end-product or by-product inhibited, while in other cases the growth rate of the bacteria is end-product or by-product inhibited. Still in other cases, the range of suitable feedstocks is narrow, or the production of undesirable gases is high. In some processes, the rate of hydrogen gas production is very low, while in all cases faster rates are highly desirable.
For example, photosynthetic processes exist (U.S. Pat. No. 5,804,424 to Kaplan et al; U.S. Pat. No. 4,921,800 to Vatsala, and U.S. Pat. No. 4,919,813 to Weaver) whereby photosynthetic organisms (such as algae or microalgae) use light to convert water into hydrogen gas and oxygen gas. However, the yields of hydrogen gas are very low in these instances because one of the end products of the reaction, oxygen gas (O2), Irreversibly inhibits the hydrogenase enzyme responsible for hydrogen gas production. Even a very small amount of oxygen gas produced as a byproduct is sufficient to shut down the entire hydrogen gas production system by inactivating the hydrogenase enzyme. Thus, the amount of hydrogen gas produced is minimal, rarely exceeding 10 ppm. Efforts to prevent this oxygen gas inhibition by modification of the hydrogenase enzyme have met with only modest success. These altered hydrogenases can only tolerate 2% oxygen gas concentration.
Pond scum has been utilized recently to produce hydrogen gas at a rate of 3 ml/l-hour. A. Melis, et al., “Sustained Photobiological Hydrogen Gas Production Upon Reversible Inactivation of Oxygen Evolution in the Green Alga,” Proceedings of the 1999 U.S. DOE Hydrogen Program Review, pp. 1-19. However, this was accomplished only while maintaining an oxygen-free environment for the algae and only for short time spans up to four days.
Another approach to producing hydrogen gas is the process of fermentation. Processes utilizing monocultures of various mesophilic bacteria produce hydrogen gas as a byproduct of anaerobic fermentative degradation of simple sugars (Roychowdhury, U.S. Pat. No. 4,480,035) or a combination of either formic acid or a formate plus a nitrogen source (Sanford, U.S. Pat. No. 5,834,264). Attentively, glucose or glucose-containing polysaccharide feedstocks are utilized (Taguchi, U.S. Pat. No. 5,350,692). These processes confer advantages over photosynthetic processes because fermentative processes use less water, require no direct input of solar energy, and eliminate the need for a container with a large translucent surface area. Fermentative mesophilic bacteria only need an input stream consisting of an appropriate aqueous medium and substrate and an output stream to remove the generated waste products and gases. Unfortunately, monocultures of mesophilic bacteria may be easily contaminated. This is a major drawback in an industrial-scale fermenter where complete sterilization is difficult and where contamination is almost unavoidable.
A variation of the fermentation approach has been to co-culture a number of different types of bacteria, where the net effect is to produce hydrogen gas as a by-product of fermentation of various carbohydrates or even sludge. (Ueno, U.S. Pat. No. 5,464,539). Co-culture with photosynthetic organisms has also been described. (Weaver, U.S. Pat. No. 4,919,813). Some of these processes produce noxious gases, such as hydrogen sulfide (H2S) and methane (CH4), along with other gases that would need to be separated from the hydrogen. Composting (i.e., partial decomposition) may be required as a time-consuming preliminary step to predigest the Initial feedstock and form more simple compounds that can then be utilized to produce hydrogen.
Attempts have been made to utilize mesophilic and thermophilic bacteria in hydrogen gas production processes. (See for example, M.W.W. Adams, CHEMTECH, November 1991, pp. 692-699.) Such bacteria as Pyrococcus furiosus, Pyrodictium brockii and Thermotoga matitima were examined. However, the hydrogenase system of the P. brockii and T. maritima showed a preference for H2 oxidation, versus H2 evolution. Also, the thermostability of T. maritima enzyme appeared inferior vis-a-vis P. furiosus. 
A need exists in the art for a biological process for generating hydrogen gas on an industrial scale. The process should utilize a wide variety of hydrocarbon sources. Furthermore, the process should maximize hydrogen gas production and minimize hydrogen sulfide gas production. The process should have an inherent feature for minimizing its contamination.